Post groyne construction monitoring, Poole Use of CCO survey data 12 December 2012 Jon Kemp

Introduction This presentation consists of two parts: Numerical modelling of Sandbanks to Branksome coastal defences (2002 – 2009) Study site and challenges Study approach Numerical modelling Post construction monitoring (2009 – present day) Assess performance of groynes Compare beach widths either side of rock and timber groynes Assess beach widths to the west of the new groynes Compare modelled to measured beach widths Review beach survey programme 12 December 2012 CCO Annual Partners Meeting

Study site and challenges Narrow beaches Overtopping Scour and undermining of seawalls 12 December 2012 CCO Annual Partners Meeting

Study approach Literature review Gather guidance on beach control schemes – both UK and abroad Included sub-contracted reviews (from New Zealand and Italy) Review of current standard of defence and beach conditions along study frontage and coastal processes Initial numerical modelling of beaches/ groynes Initial short-list of suitable options drawn up Multi-criteria analysis to assess direct and indirect benefits of 10 short-listed options Numerical modelling of 4 options from the MCA and recommend a preferred option Refinement of preferred option – numerical modelling 12 December 2012 CCO Annual Partners Meeting

Beach modelling – initial scheme 12 groynes: 75m long at 225m spacing Aims of numerical modelling Comparison of four Phase 1 options Maintaining adequate beach widths Efficiency at retaining beach recharge Impact on downdrift beaches Relative comparisons Recommend a “final” preferred scheme Refinement of scheme 12 December 2012 CCO Annual Partners Meeting

Beach modelling – refined scheme Only 5 groynes: 75m long (or less) at 150m spacing No groynes to the west 2:1 groynes worked best Another important conclusion was that there was no need for any structure along the western section – with groynes installed led to localised narrowing More cost effective to undertake direct improvements to the seawall, expect some narrowing over time + further recharge Big cost saving and better amenity and aesthtic value 12 December 2012 CCO Annual Partners Meeting

Completed scheme – May 2009 Photos courtesy of PBC 12 December 2012 CCO Annual Partners Meeting

Post construction monitoring 5 year post construction monitoring study Post construction monitoring (2010/11 – 2014/15) Assess performance of groynes Compare beach widths either side of rock and timber groynes Assess beach widths to the west of the new groynes Compare modelled to measured beach widths Review beach survey programme Work to date Year 1 (FY2010/11): Analysis of beach survey data along the Poole frontage up to June 2010 (HR Wallingford, 2010). Year 2 (FY2011/12): Analysis of beach survey data along the Poole frontage up to June 2011 (HR Wallingford, 2011). 12 December 2012 CCO Annual Partners Meeting

Available survey data   12 December 2012 CCO Annual Partners Meeting

Available survey data 12 December 2012 CCO Annual Partners Meeting

Management and analysis of survey data Data Management Beach Data Analysis system (BDAS) Generation of digital elevation models (DEMs) using ArcGIS Data Analysis Shoreline position – MHW (0.5 m OD) contour extracted from DEMs Beach width – Statistical analysis of MHW contour positions at all CCO profiles Cross-sectional area – Good indicator of the “health” of a beach. Extracted from CCO annual reports 12 December 2012 CCO Annual Partners Meeting

Management and analysis of survey data 12 December 2012 CCO Annual Partners Meeting

Shoreline positions 15 m increase Short-term variations of ±5m Seasonal changes in beach levels Drift reversals Groynes reached capacity Effective in slowing longshore drift 15 m increase Groynes 4 5 and 6 have reached capacity For as long as there is sufficient sand supply from the west it is likely that any sand being transported eastwards (dominant direction) will no bypass the groynes June 2010 shows westward drift accretion against eastern side of groynes Overall the groynes have been effective in slowing down the drift – as the supply of sand diminishes the effectiveness of the groynes in slowing drift and retaining sand will become more apparent 12 December 2012 CCO Annual Partners Meeting

Change in beach levels in this particular image shows a steepening of the beach- lowering of levels offshore of MHW and increase in levels landward of this Result of natural re-profiling - This analysis also shows seasonal fluctuations – Also Show an increase at back of beach by up to 0.5m (wind blown sand now a minor problem) However from this type not possible to judge whether the volume has changed or if it is just a redistribution vertically 12 December 2012 CCO Annual Partners Meeting

Change in beach width General increase in beach widths but quite a lot of short-term variation Over time the comparison of the variability in beach widths between the two types of groyne will become more meaningful. Will also be interesting to see whether the short-term variations in response to different wave directions, storms and variations in slopes will be any different for the different groyne types 12 December 2012 CCO Annual Partners Meeting

Change in cross-sectional area Considering only beach widths alone can sometimes be misleading – short-term changes in the slope often occur in response to a storm as well as seasonally with the result that the width of the upper beach will alter but the overall volume (per metre run of frontage) might be the same. Volume of beach is a more reliable indicator of the health of the beach – on a beach that has been recharged and will likely be need to be recharged in the future – studying the changes in volume is particularly important. To investigate change in volume we used the CCO annual reports Instead of repeating the analysis of beach profile data Western end Reduction in volume at western end is inevitable due to net eastwards longshore drift Expected that this pattern of erosion will continue into the future – indicating that the sand arriving is smaller than volumes being carried eastwards Could become a concern at Flag head chine first because of seaward bulge in the plan shape of the seawall Eastern end Losses have been less indicating the groynes have been working Previous problems of low beach levels, particularly in front of the seawall at Branksome Dene change have been solved at least for the immediate future The table also suggests no localised beach erosion has occurred immediately to the east of the new rock groynes. Western end - Gradual decrease in sand along western part of frontage Eastern end – Less sand loss and haver stabilised in last three years 12 December 2012 CCO Annual Partners Meeting

Comparison with modelling results Main differences are in the immediate lee of the groynes Note that the picture shown is just a snapshot in time using hindcast wave data – the waves since 2009 are likely to be different than those modelled We are now in the process of remodelling the beach evolution with new wave data measured by cefas and CCO 12 December 2012 CCO Annual Partners Meeting

Conclusions Monitoring data Groyne performance Western frontage Useful resource for future studies of beach management and fundamental research into beach and groyne processes Following recommendations from first report CCO changed the survey schedule to co-ordinate Poole and Bournemouth surveys Groyne performance Effective in slowing longshore drift and maintaining beach widths. Not yet enough data to make robust conclusions on model vs. measured or timber vs. rock groynes Western frontage Beach width has decreased and rate of loss is in line with the modelling No evidence to suggest the decision not to install groynes was unsound 12 December 2012 CCO Annual Partners Meeting